1. Field of the Invention
The present invention relates to a semiconductor device having an overcurrent protective circuit that detects the flow of excessive main current (overcurrent) exceeding a predetermined limit in a switching element, and more specifically to a semiconductor device that can regulate the operation level of a overcurrent protective circuit without changing the reference voltage.
2. Background Art
A semiconductor device that drives a switching element, such as an insulated gate bipolar transistor (IGBT) element, and also has an overcurrent protective circuit that detects the flow of excessive main current (overcurrent) exceeding a predetermined limit in a switching element, has been used in power converters, such as a DC/DC converters and inverter devices.
A semiconductor device wherein switching elements, switching device driving functions, overcurrent protective functions and the like are integrated is called an IPM (intelligent power module). All the semiconductor devices having IPM functions, DC link capacitors, on/off signal generating functions of the switching elements (inverter control), means to detect the state of load (various sensors) and the like are called inverter devices.
In IPMs or the like, the overcurrent protective function is realized by special ASIC or hardware (H/W) such as electronic circuits. Although the overcurrent protective function by H/W has an advantage of high response speed to unusual generation of overcurrent or the like, it has a disadvantage of low current detection accuracy. Therefore, there is a problem of large variation of overcurrent levels at which the overcurrent protective circuit operates.
A general inverter device has relatively accurate current detecting means, such as a current transformer and a hole element, to detect output current. The output current of the current detecting means is inputted into a microcomputer of the inverter controlling section or the like via an AD converter. When the program (software) implemented on the microcomputer (MPU) detects an overcurrent, the program takes protective action, such as stopping the operating of the inverter. Although the protective function by software (S/W) has an advantage of higher current detection accuracy than the protective function by H/W of IPMs, it has a disadvantage of low response speed. Therefore, there is a problem of delay in protection leading to the failure of the switching element or the damage of the load.
Here, the reason why the current detection accuracy of overcurrent protective functions by IPMs or the like is low will be described. In IPMs or the like, to realize overcurrent protective functions, a switching element, such as IGBT and MOSFET, is provided with a current sensing element, to use for current detection. The current sensing element has a cell structure identical to the cell structure of the switching element, connected to the switching element in parallel, and has a constant cell area to the switching element.
FIG. 12 is a diagram illustrating an equivalent circuit of a switching element that incorporates a current sensing element Under ideal operation conditions, Area ratio (Cell area of current sensing element/Cell area of switching element) agrees with Shunt ratio (Sense current/Switching element current) of the switching element and the current sensing element to become Sense current (Switching element current×Cell area of current sensing element/Cell area of switching element).
The overcurrent protective circuit performs protective operations, such as turning off the switching element, when the sense current value flowing through the current sensing element exceeds a predetermined limit. Actually, the sense voltage wherein the sense current value is converted to a voltage and the reference voltage are inputted in a comparator, and the output of the comparator is made to be the operation initiating trigger. Conventional semiconductor devices wherein the sense current value is thus converted into a voltage are shown in FIGS. 13 and 14.
The semiconductor device shown in FIG. 13 has a current-voltage converting circuit using an operational amplifier. In this case, since the portion of the operational amplifier between the −terminal and the +terminal is in an imaginary short-circuit state, the operation of the current sensing element is not affected. However, in a general power converter, since the changing rate per hour of the switching element current is as large as several kA/μs and the changing rate of the sensing element current is also large, delay in response of the operational amplifier cannot be ignored, and accurate current-voltage conversion cannot be achieved. Furthermore, since the obtained sense voltage is a negative value, two power sources of positive and negative including the overcurrent protective circuit were required, and the size reduction of the device became difficult. Therefore, the circuit shown in FIG. 13 is little used.
The circuit shown in FIG. 14 employs a simple method wherein the voltage drop of the sense resistor is treated as the sense voltage, and does not require an operational amplifier having a high-speed response as the circuit shown in FIG. 13. Therefore, the circuit shown in FIG. 14 has been heretofore used. However, since the circuit shown in FIG. 14 has the problems described below, current detecting accuracy is low, and current levels of the switching element at which the overcurrent protective circuit operates is substantially varied.
Firstly, since the gate-emitter voltage of a current sensing element equals to (gate-emitter voltage of a switching element)−(sense voltage), the gate-emitter voltage VGE of the switching element does not equal to the VGE of the current sensing element. Similarly, the collector-emitter voltage VCE of the switching element does not equal to the VCE of the current sensing element. Therefore, the relationship of (Cell area of current sensing element/Cell area of switching element=Sense current/Switching element current) is not established. Further, since the gate threshold value of IGBT or MOSFET has temperature characteristics, the effect of the difference between VGE and VCE of the switching element and the current sensing element on (Shunt ratio=Sense current/Switching element current) varies depending on element temperatures. This means that the shunt ratio is varied by element temperatures. Therefore, conventional semiconductor devices had a problem wherein when the element temperature of the switching element was varied, the operation level of the overcurrent protective circuit was also varied.
A semiconductor device solving this problem has been proposed (for example, refer to Japanese Patent Laid-Open No. 2005-151631; FIG. 2). In the circuit described in Japanese Patent Laid-Open No. 2005-151631; FIG. 2, a sense voltage (output S4 of a maximum value retaining circuit 232) and a reference voltage (output REF of a reference voltage generating circuit 233) are inputted in a comparing circuit 234. When overcurrent flows in a switching element 1, the output S2 of the comparing circuit 234 performs protecting operations, such as the activation of the drive circuit 211 to turn the switching element 1 off. In this circuit, by making the reference voltage REF variable according to the temperature of the switching element 1, it is possible to maintain the operation level of the overcurrent protective circuit constant even if the element temperature of the switching element 1 varies.
In the circuit described in Japanese Patent Laid-Open No. 2005-151631; FIG. 2, in order to minimize the effect of the sense voltage in the shunt ratio of the switching element current and the sensing element current, it is required to reduce the value of the sense resistor as much as possible to reduce the sense voltage and the reference voltage. However, since the comparing circuit is operated by one-side power source, if the reference voltage is lowered to excessively close to the GND level, the circuit cannot operate. Therefore, since the width of the variable range of the reference voltage corresponding to temperature change could not be widened, the operation level of the overcurrent protective circuit could not be adjusted within a sufficiently wide range. Furthermore, if the reference voltage of the overcurrent protective circuit was lowered to lower the trip level, the malfunction by noise overlapped with the sense voltage caused a problem.
In addition, a reference voltage generating element, such as a zener diode and a band gap reference having favorable temperature characteristics, is generally used in a reference voltage generating circuit. Although it is difficult to make the voltages of these reference voltage generating elements variable, the specific method to vary the reference voltage is not disclosed in Japanese Patent Laid-Open No. 2005-151631; FIG. 2.